My girlfriend allows another guy to lay his head on her breasts and lap, what does it mean ?

Me and my girlfriend have been together for six months.We are 18, we are in different classes.The problem is that she has a friend in her class who is really close to her, she says he is her best friend and they know each other for years. He is a short guy who has the nasty habit of resting his head on my girlfriend`s lap and even on her breasts whenever he wants and she allows him to do that. I saw them in the school yard, she was reading a book and he was lying on her lap and then he moved his head on her belly. I went to them and started shouting what were they thinking. Then she was mad at me, she said he is her best classmate and i should not behave that way because they are long time friends. I agreed she was right. He is very short, much shorter than my girlfriend and I doubt she finds him attractive, but she is very beautiful so maybe he likes her. There was another case later, when in the break I entered their class room they were sitting next to each other and he has leaned on her and he was resting his head on her breasts. This time I did not say anything because I remembered her reaction the previous time, but I totally did not like the sight how her breasts were squeezed under the weight of his head. Just later I was a bit grumpy and that`s it. What should I do, I do not like the way this little short brat uses my girlfriend`s boobs and lap as a pillow and hugs her, but she hates it when I speak bad things about him. Do you think she enjoys his head buried in her intimate areas, or he is just too impudent?